Perhaps an evil wizard who has a vendetta against all dragons. He sees Dragon Symmetra as a key to his plan of destroying the dragon race. He uses dark magic to try and corrupt her or stop her from using her powers for good. He could be constantly setting traps and sending minions after her to achieve his evil goals.
Another possible main antagonist could be a group of dragon slayers who don't understand her symmetra powers. They think all dragons are bad and see her as an even greater threat because of her unique abilities. They are constantly hunting her down, and she has to find a way to either convince them she's not a threat or defeat them to protect herself and her dragon kind.
Her main antagonist could be a power - hungry dragon who wants to steal her symmetra powers for himself. This dragon might be jealous of the unique abilities she has and sees it as a way to gain ultimate control over the dragon world.
The World Government could be the main antagonist in a more complex way. They could be secretly manipulating events to pit Dragon and Luffy against each other. For example, they could spread false rumors about Luffy being a tool of the revolutionaries, which would make other pirate crews and kingdoms target him, and at the same time make Dragon look bad for having a son who is causing so much chaos. This would force Dragon and Luffy to either fight each other or find a way to clear their names together.
It could be Morgana. She might see Merlin's transformation into a dragon as a threat to her own power and plans. So, she tries to stop him from fully becoming a dragon or tries to use his new form for her own evil purposes.
It could be a power - hungry wizard who wants the dragon for himself. He might try to steal the egg from Jon before it hatches or take the newly - hatched dragon by force.
Perhaps a jealous wizard. He was once friends with Astrid but when he sees her gain the power and beauty of a dragon, he becomes envious. He starts to cast spells to make Astrid's life as a dragon difficult and tries to steal her new - found powers for himself.
It could be a character who is jealous of Naruto's new dragon powers. Maybe someone who has always been in competition with Naruto and now feels even more threatened by his enhanced abilities.
A powerful and corrupt World Noble could be the main antagonist. They might be trying to stop both Luffy and Dragon's efforts for freedom. The World Noble could have a large military force under their control and be causing chaos and oppression in various parts of the world. This would force Luffy and Dragon to join forces to take them down.
Another option is a rival dragon slayer who sees Natsu's transformation as a threat to his own status. This rival could try to stop Natsu from fully becoming a dragon or try to defeat him to prove his own superiority.
A new character could be created as the main antagonist. For example, a powerful sorcerer from a far - off land who has come to Hyrule seeking its magical energy. He wants to use the power of the Dragon Knight to open a portal to a realm full of dark magic.
Frieza could be a great antagonist. Since Raditz is now part of the Dragon Team, Frieza might see this as a threat and decide to target Earth. His powerful forces and his own desire for universal domination would put him in direct conflict with the Dragon Team and Raditz, who has a personal vendetta against Frieza for the oppression of the Saiyans.
An ancient dragon that has turned evil due to some dark magic could be the main antagonist. This dragon might see Natsu as a threat because of his unique combination of powers and wants to eliminate him. Natsu would then have to use all his skills and the help of his friends to fight this dragon. The dragon could also have minions that are constantly causing trouble for Natsu and his guild.